A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In general, in the optical system of a lithographic apparatus reflectors, such as mirrors, are used to reflect a beam of radiation, in order to shape or focus the radiation beam. In lithographic apparatus using EUV radiation, mirrors are used, for example, to shape the radiation beam and manipulate the radiation beam otherwise. However, mirrors have limited reflectivity. Thus, not all of the incident radiation is reflected by the mirror but a part of the incident radiation is absorbed by the mirror. The absorbed part of the incident radiation energy is converted into heat, which gives rise to a temperature increase of the mirror. This increase in temperature may have a negative effect, such as an unwanted deformation of the mirror. In a systems using one or more mirrors, the effects of deformation of the mirrors on the optical performance are larger compared to systems using defractive options, such as lenses. Accordingly, in a lithographic apparatus using mirrors, the thermal aspects of the mirror(s) need specific attention.
It is known from the Patent Abstracts of Japan 2002-100551 to provide a reflector with a piezoelectric element that can apply a compensating force to the reflector to correct the thermal deformations. However, the piezoelectric element has the disadvantage that complex and expensive (piezo)electric circuitry has to be provided.
Another manner of counteracting thermal deformation of a reflecting mirror is known from the Patent Abstracts of Japan 2000-036449. In this prior art document, a mirror with heating elements positioned against a back of the mirror is described. The mirror is selectively heated by the heating element, in order to provide a uniform heating of the mirror, and thus reduce thermal deformations of the mirror. However, this solution requires a complex system of control circuits and heating devices to apply an appropriate heating.
Moreover, in both prior art devices, only the local deformation is prevented.